Extensometer



June 29 1937. o. s. PETERS EXTENSOMETER Filed Jan. 11, 1933 y Patented June 29, 1937 PATENT -oFFlca EXTENSOMETER Orville S. Peters, ChevyChase..Md., assigner to l.faldwin-Southwax'k Corporation, Philadelphia,

" `Application .Ianuary 11,51933, SerialNo. 651,205

9 claims. (ci. asf-147) This invention relates generally to Van improved extensometer for determining the strain of a specimen subjected to' stress.

` An extensometer must be adapted to measure accurately very minute changes in the `strain of a specimen without subjecting `the gauge points between the specimen and extensometer to ab normal stresses, which would cause slippage and thus prevent a true measurement being obtained.

1.0 It is also desirable that the extensometer should be rugged, combined with the foregoing sensitivity, and should be economic-al to manufacture while retaining the scientific aspects ofthe instrument.

` `It is one object of my invention to accomplish the foregoing results in an improved extensometer and more particularly an averaging extensometer. In a more `specific aspect of the invention, it is an object of my invention to provide an improved extensometer frame constructed of very vsubstantial material and yet arranged fora high degree of flexibility. A further object is to combine with such a frame improved electrical contact operating means together with micrometer adjustment for measuring the variations in strain of a specimen.

Other objects and advantages will be moreapparent from the following description of the accompanying drawing in'which: 3 Fig. l is a perspective of my improved extensometer which is applied to a fragmentary portion of a specimen;

Fig. 2 is a side elevation of the extensometer as applied to the specimen; Fig. 3 is a horizont-a1 section taken substantially on the line 3-3 of Fig. 2;

Fig. 4 isa vertical section taken substantially y on the line 4--4 of Figs. 2 and 3;

` Fig. 5 is a vertical section taken substantially on the line 5-5 of Fig. 2 showing means for effecting a vdefinite gauge length when the extensometeris being secured to the specimen;

Fig. 6 is a fragmentary view of a modified form of the extensometer frame.

`The illustrated embodiments of the invention shown herein are a few among other possible formswhich the invention may take, and comprise a substantially U or C-shaped extensometer frame generally indicated at I having two substantially parallel side legs 2 and 3 connected by a closed end or end leg 4, these legs being formed of fiat rectangular metal which is relatively wide compared to `its narrow thickness. The thickness is preferably approximately onefourth the width, although it will of course be -desired degree of reduction at I4. The side legs understood that different proportions may be employed depending upon particular circumstances and the quality of metal which preferably is of mild steel. The sides 2 -and3 and end 4 are of comparatively heavy normally inflexible material formed along substantially straight lines at right angles to each other. .Sides 2 and 3 have gripping heads 5 and 6 formed preferably integrally therewith as by being machined from a solid piece of material, the heads having preff` 10 erably greater axial length than the thickness` of said sides. These heads as shown in Fig..3 have a suitable recess 'I of U-shaped or other form to receive a specimen 8. To permit maximum flexibility of the foregoing `rugged frame 15 and to obtain an average of any unequal deformations of the specimen, the frame is relieved as by transverse grooves Iii and II on opposite sides of each of the legs 2 and 3, the combined depth of the two opposite grooves being such as to provide a substantially reduced portion or web I2. It will of course be understood that various lshapes of grooves may be employed to effect this desired degree of reduction or if desired this reduction may be effected by having the opposite surfaces of each leg taper toward each other vsomewhat in the manner employed at the corners 2 and 3 and end 4 are preferably pre-formed from an integral piece of metal such as steel and bent into its U-shape as shown, although' if desired the sides 2 and 3 `and end 4 may be formed from 40 separate members as shown in Fig. 6 and then butt welded, soldered or otherwise suitably secured together along vertical planes of juncture Il, it being noted that in this Fig. 6 form the corners ofthe frame generally indicated at I are provided with opposed relief recesses I4 and I6 but the frame is otherwise identical to the preferred form. p

The frame is secured to the specimen by a pair of upper gauge points I8and I9 and a pair of lower points 20 and 2l, points I9 and 2l being threaded screws 'while points I8 and 20 as shown in Fig. 3 are suitably embedded in one side of the recess in heads 5 and 6 either by a pressed fit or if desired by a. screw thread. It will be noted 5 that the opposed gauge points are oiset from the center of the frame, thus permitting ample size recesses I in the gripping heads while keeping the heads the same width as the side legs of the frame. To obtain a uniform gauge in applying the extensometer to any specimen, there is provided a spacing or gauge sleeve 22 herein shown as secured to the lower head 5 by any suitable means such as welding, soldering or the like. Sleeve 22 isof a definite length so that when the upper arm' 3 is pressed toward arm 2, sleeve 22 will engagehead 6 to provide the denite gauge between the gauge points, at which time the adjustable points are secured to the specimen. If desired a rod l23 may extend through sleeve 22 and aligned openings in heads 5 and 6, the lower end of said rod carrying a stop head while the upper end pivotally carries a small lever actuated cam 24 engageable with the top surface of head 6. Rotation of said cam will cause the head of rod 23 to engage head 5 and thus positively hold heads 5 and 6 in engagement with sleeve 22. This will permit the frame to be applied to the specimen. or the extensometer when so clamped may be carried in a suitable container for transportation or safekeeping with minimum danger of injury.

'I'he strain indicating mechanism comprises a lever system having micrometer adjusted electrical contacts with means for obtaining the average elongation of the specimen or the elongation at the axis thereof. This system includes a plate 26 held by and in spaced relation to the upper gripping head 6 by screws 21 extending through said plate and through a pair of spacer sleeves 23 for threaded engagement with said head. To provide a pivot bearing for the inner end of an averaging pin 29 whose outer end is pivotally supported in plate 26 and also to insure that the specimen will be properly centered with respect to the gauge points, a centering pin 29 is inserted through the rear side of head 6 and projects sufiiciently into recess 1 to engage and center specimens of the same diameter. A similar centering pin is also provided in head 5. Pin 29 supports a depending arm 30 normally urged toward the frame by a spring 3I connected to pins 32 and 33 carried respectively by arm 30 and the lower head 5. A bellcrank lever 34 has a horizontal knife edge 34a engaging a suitable V recess in the lower 4end of arm 30 while an averaging pivot 34" seated in the rear side of lower head 5 has its axis disposed to intersect that of a specimen. The upper end of said lever 34 carries a contact 35 engageable with the end of an adjustable micrometer screw 36. The micrometer generally indicated at 31 has a usual stationary housing 38 provided with a flange 39 and a reduced cylindrical portion ,adapted to project into an opening 46 formed within an enlarged boss on arm 30. Suitable insulation 4I is disposed in opening 40 between the micrometer and arm 30 while further insulation 42 is interposed between flange 39 and arm 30. The micrometer housing 33 is secured to arm 30 as by a series of screws 43, passing through flange 33 and threaded in insulation 42 which in turn may beheld by other screws (not shown) threaded into arm 30.

An electrical wire 43 is connected, as by a nut 44, to contact 35 which is insulated from its arm 34, this wire leading down to one of two binding posts 45 while the other binding post is suitably connected by a similar wire (not shown) over to the ange 39 of the micrometer housing, these wires being suicientlyexible so as not to interfere with the free movement of the bellcrank lever.

In operation, the free ends of the frame are pressed together, either by hand or by cam 24, into engagement with spacer sleeve 22 and then secured to the specimen by the two sets of gauge points. Suitable wires from any desired electrical recording mechanism are then connected to the binding posts 45 and the micrometer 31 rotated until its contacts 35 and 36 engage thereby causing transmission ofv electric current to the recording apparatus. As the specimen elongates under tensile stress, heads 5 and 6 will move away from each other, thus causing pivot 34 to move bellcrank lever 34 around its knife edges 34a away from micrometer carrying arm 33, thus breaking engagement between contacts 35 and 36 whereupon the micrometer stem is further adjusted to reestablish engagement between said contacts. These contacts may be maintained in engagement either manually or automatically depending upon the-type of recorder mechanism. In the event that automatic micrometer adjustment is effected, a pulley 46 provided on the micrometer head is rotated by a belt leading from any rotatable restoring mechanism of the recording mechanism. As the specimen elongates the extensometer frame will readily yield at its relieved portions I2 and I4 without subjecting the gauge points to abnormal stresses, it being understood that if an abnormal stress or load should be imposed upon these points the accuracy of the strain measurement may be materially interfered with due to slippage between the points and specimen. Due to the axes of the upper and lower sets of gauge points extending longitudinally of arms 2 and 3,'it is seen that if one side of the specimen should elongate more than the other side that only the average of these two elongations would be transmitted to contacts 35 and 36 because the axes of averaging pins 29 and 34 intersect the axis of the specimen. In the event of any difference in elongation on the two sides of the specimen, which may occur due to unequal loading, the gauge points will not be subjected to undue pressure because the reduced portions I2 adjacent the upper and lower heads 5 and 6 will permit these heads to assume such inclination that the opposed gauge points may follow the elongation on their respective sides. The reduced portions I2 and I4 will also allow for full compensation of any distortion to which the frame may be subjected during differences in elongation on opposed sides of the specimen, and these reduced portions will also permit maximum flexibility and sensitivity of the frame throughout a strain test.

As a result of the foregoing operation of the gauge points and relieved portions, the heads 5 and 6 remain more nearly normal to the specimen axis during fiexure of the frame than if the gauge points were so arranged that a direct pivoting relation might occur between the frame and specimen. Hence lever 34 is responsive directly to strain of the specimen substantially independently of flexure of the frame, thus insuring maximum accuracy and sensitivity. A further advantage of my improved extensometer is in having the axis of pivots 29 and 34 substantially parallel and preferably substantially midway between the gauge points, whereby lever 34 and arm 33 will be maintained in their predetermined normal relation in the event that either head tends to pivot against the holding force of their gauge points. Such pivotal movement of the heads will not displace the lever and arm unless the pivotal movestrain o! the specimen.

ment the result of variation in the average From the foregoing it is seen that I have provided a relatively simple extensometer which is y extremely rugged andtyet combines therewith a high degree of ilexibility, although it will of course be understood that those skilled in the art may obtain my results by making various changes in the elements and rearrangements thereof without departing from the spirit of the invention as set forth in the appended claims.

I claim:

1.l An extensometer' comprising a 'substantially U-shaped frame, one end of said frame having opposed `gauge points for attaching the frame to a specimen, means for operatively connecting l `the other end of the frame to the specimen, and

means for indicating elongations of the specimen including a member pivotally engageable with cluding a leverage system having, a plurality of pivots for transmitting elongations of the specimen from the frame, the axes of vsaid pivots `lying at longitudinally spaced points of a specimen and substantially midway between said gauge points whereby an average elongation of the specimen is obtained.

a leverage system having a vertical arm extending substantially parallel to the specimen and a pluralityof pivots for supporting said arm by said by said frame and including a depending pivotally supported arm carried by one of said heads,`

a bellcrank leverV pivotally supported by the free end of said arm and by theother oi said heads and projecting upwardly toward the pivot from which said arm depends, and a micrometer secured to said arm and having an adjustable stem projecting through said arm for making adjust-` able contact with the free end of said lever.

5. The combination set forth in claim 4 further characterized in that the pivotal support for said arm includes a plate secured to and spaced from one of the heads of said parallel legs, and `a pin pivotallysupported by said plate to carry said arm.

6. An extensometer comprising a substantially U-shaped frame, means for attaching the free ends thereof to axially spaced points of a specimen, and strain measuring means carried by said frame and including an arm pivotally supported frame, the axes of said pivots lying respectively' substantially midway between said pointswhereby an average elongation of the specimenisobtained electrical contacts carried by said leverage system,.and a micrometer supported by said arm for effecting adjustmentbetween said contacts.

l men, an electrical contact leverage system carried by said frame about an axis transverse thereto, and a lever pivotally supported by said arm about an axis transverse to said other axis for transmitting elongations of the specimen from the frame.

7. An extensometer comprising a substantially U-shaped frame, means for attaching said frame to axially spaced points of a specimen. and strain measuring means including an arm pivotally supported by said frame about an axis transverse to' the frame, a lever also pivotally supported by said frame about an axis substantially parallel to said other axis, and means for pivotally supporting said lever by said arm about an axis transverse to said other axis.

8. The combination'set forth in claim 7 further characterized by the provision of yieldable means for holding said lever pivots under compression.

9. An extensometer comprising a frame, means for attaching said frame to axially spaced points of a specimen, strain measuring in eans including an arm pivotally supported by said frame about an axis transverse thereto, a lever, means for pivotally supporting said lever by said arm, and means for also pivotally supporting said lever by said frame about an axis substantially parallel to said transverse axis whereby said lever and arm 

